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Best definition of a 'discrete molecule'... - HSN forum

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Best definition of a 'discrete molecule'...


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#1 natalie1062

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Posted 20 September 2004 - 02:37 PM

I know it's a simple concept but what is the best definition of a discrete molecule?
And also why do such molecules have relatively low melting/boiling points?
Tia.

#2 George

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Posted 20 September 2004 - 03:12 PM

Excuse me if I'm a bit rusty unsure.gif

If you check out the notes for Topic 3 - Patterns in the Periodic Table, on page 5 you'll see a group of elements labelled "discrete covalent molecules".

Basically, covalent bonds hold each molecule together. The molecule is made up of atoms held together by covalent bonds.

Melting/boiling points are determined by intermolecular forces. With discrete molecules, the only such forces are van der Waals forces, which are weak hence relatively low melting/boiling points.

Hope that helps smile.gif

#3 Ally

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Posted 20 September 2004 - 03:20 PM

Yeah it's exactly what George has said.

Basically a 'discrete molecule' is a covalent molecule in which the intermolecular forces are really weak, hence the low melting and boiling points of these molecules.

The forces which act between these molecules are called Van Der Waals forces which is a intermolecular (between molecules) force. Since Van Der Waals forces are weak, only a small amount of energy is needed to break apart the molecules - so low m.p and b.p.

Make sure you don't get intermolecular forces confused with intramolecular forces, which hold an individual molecule together. They can be easily confused!

Van der Waals forces exist between all atoms and molecules.

Here is a diagram of Van Der Waals forces acting between a molecule:

user posted image.


Here is another diagram showing how these Van Der Waals forces arise:

user posted image

Hope that is of any help!

#4 natalie1062

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Posted 20 September 2004 - 03:33 PM

Thanks guys, much appreciated biggrin.gif





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